diff --git a/app/controllers/activities_controller.rb b/app/controllers/activities_controller.rb
index 7746ec2f6..cf70bdfd7 100644
--- a/app/controllers/activities_controller.rb
+++ b/app/controllers/activities_controller.rb
@@ -32,13 +32,15 @@ class ActivitiesController < ApplicationController
     @date_to ||= User.current.today + 1
     @date_from = @date_to - @days
     @with_subprojects = params[:with_subprojects].nil? ? Setting.display_subprojects_issues? : (params[:with_subprojects] == '1')
-    if params[:user_id].present?
-      @author = User.visible.active.find(params[:user_id])
+    @authors = []
+    if params[:user_ids].present?
+      @authors = User.visible.active.where(:id => params[:user_ids])
+      raise ActiveRecord::RecordNotFound if @authors.empty?
     end
 
     @activity = Redmine::Activity::Fetcher.new(User.current, :project => @project,
                                                              :with_subprojects => @with_subprojects,
-                                                             :author => @author)
+                                                             :authors => @authors)
     pref = User.current.pref
     @activity.scope_select {|t| !params["show_#{t}"].nil?}
     if @activity.scope.present?
@@ -47,7 +49,7 @@ class ActivitiesController < ApplicationController
         pref.save
       end
     else
-      if @author.nil?
+      if @authors.empty?
         scope = pref.activity_scope & @activity.event_types
         @activity.scope = scope.present? ? scope : :default
       else
@@ -62,7 +64,7 @@ class ActivitiesController < ApplicationController
         @activity.events(@date_from, @date_to)
       end
 
-    if events.empty? || stale?(:etag => [@activity.scope, @date_to, @date_from, @with_subprojects, @author, events.first, events.size, User.current, current_language])
+    if events.empty? || stale?(:etag => [@activity.scope, @date_to, @date_from, @with_subprojects, @authors.first, @authors.size, events.first, events.size, User.current, current_language])
       respond_to do |format|
         format.html do
           @events_by_day = events.group_by {|event| User.current.time_to_date(event.event_datetime)}
@@ -70,8 +72,8 @@ class ActivitiesController < ApplicationController
         end
         format.atom do
           title = l(:label_activity)
-          if @author
-            title = @author.name
+          if @authors.any?
+            title = @authors.map(&:name).join(", ")
           elsif @activity.scope.size == 1
             title = l("label_#{@activity.scope.first.singularize}_plural")
           end
diff --git a/app/views/activities/index.html.erb b/app/views/activities/index.html.erb
index 83f67b1b8..1aa5edf5d 100644
--- a/app/views/activities/index.html.erb
+++ b/app/views/activities/index.html.erb
@@ -1,4 +1,4 @@
-<h2><%= @author.nil? ? l(:label_activity) : l(:label_user_activity, link_to_user(@author)).html_safe %></h2>
+<h2><%= @authors.empty? ? l(:label_activity) : l(:label_user_activity, @authors.map{|a| link_to_user(a)}.join(',')).html_safe %></h2>
 
 <p class="subtitle"><%= l(:label_date_from_to, :start => format_date(@date_to - @days), :end => format_date(@date_to-1)) %></p>
 
@@ -38,7 +38,8 @@
 </p>
 <p>
   <%= l(:label_user) %>
-  <%= select_tag('user_id', activity_authors_options_for_select(@project, params[:user_id]), include_blank: true) %>
+  <%= select_tag('user_ids[]', activity_authors_options_for_select(@project, params[:user_ids]), include_blank: true, multiple: (params[:user_ids]&.count.to_i > 1)) %>
+  <span class="toggle-multiselect icon-only"></span>
 </p>
 <ul>
 <% @activity.event_types.each do |t| %>
@@ -46,7 +47,7 @@
     <%= check_box_tag "show_#{t}", 1, @activity.scope.include?(t) %>
     <label for="show_<%=t%>">
       <%= link_to(l("label_#{t.singularize}_plural"),
-                  {"show_#{t}" => 1, :user_id => params[:user_id], :from => params[:from]})%>
+                  {"show_#{t}" => 1, :user_ids => params[:user_ids], :from => params[:from]})%>
     </label>
   </li>
 <% end %>
@@ -59,4 +60,4 @@
 <% end %>
 <% end %>
 
-<% html_title(l(:label_activity), @author) -%>
+<% html_title(l(:label_activity), @authors) -%>
diff --git a/lib/redmine/activity/fetcher.rb b/lib/redmine/activity/fetcher.rb
index 224e16492..1c52fe5ac 100644
--- a/lib/redmine/activity/fetcher.rb
+++ b/lib/redmine/activity/fetcher.rb
@@ -24,7 +24,7 @@ module Redmine
       attr_reader :user, :project, :scope
 
       def initialize(user, options={})
-        options.assert_valid_keys(:project, :with_subprojects, :author)
+        options.assert_valid_keys(:project, :with_subprojects, :author, :authors)
         @user = user
         @project = options[:project]
         @options = options
@@ -87,10 +87,15 @@ module Redmine
       def events(from = nil, to = nil, options={})
         e = []
         @options[:limit] = options[:limit]
+        authors = [[@options[:author]], @options[:authors]].flatten.compact
 
         @scope.each do |event_type|
           constantized_providers(event_type).each do |provider|
-            e += provider.find_events(event_type, @user, from, to, @options)
+            if authors.any?
+              e += authors.map { |author| provider.find_events(event_type, @user, from, to, @options.merge(:author => author)) }.flatten
+            else
+              e += provider.find_events(event_type, @user, from, to, @options)
+            end
           end
         end
 
diff --git a/public/javascripts/application.js b/public/javascripts/application.js
index 91da19229..836c56016 100644
--- a/public/javascripts/application.js
+++ b/public/javascripts/application.js
@@ -1025,7 +1025,7 @@ $(document).ready(function(){
   $('#content').on('change', 'input[data-disables], input[data-enables], input[data-shows]', toggleDisabledOnChange);
   toggleDisabledInit();
 
-  $('#content').on('click', '.toggle-multiselect', function() {
+  $('#content, #sidebar').on('click', '.toggle-multiselect', function() {
     toggleMultiSelect($(this).siblings('select'));
     $(this).toggleClass('icon-toggle-plus icon-toggle-minus');
   });
diff --git a/test/functional/activities_controller_test.rb b/test/functional/activities_controller_test.rb
index fe112da48..224e9a203 100644
--- a/test/functional/activities_controller_test.rb
+++ b/test/functional/activities_controller_test.rb
@@ -82,13 +82,15 @@ class ActivitiesControllerTest < Redmine::ControllerTest
     get(
       :index,
       :params => {
-        :user_id => 2
+        :user_ids => [2, 3]
       }
     )
     assert_response :success
 
-    assert_select 'h2 a[href="/users/2"]', :text => 'John Smith'
-    assert_select '#sidebar select#user_id option[value="2"][selected=selected]'
+    { 2 => 'John Smith', 3 => 'Dave Lopper' }.each do |user_id, user_name|
+      assert_select "h2 a[href='/users/#{user_id}']", :text => user_name
+      assert_select "#sidebar select#user_ids_ option[value='#{user_id}'][selected=selected]"
+    end
 
     i1 = Issue.find(1)
     d1 = User.find(1).time_to_date(i1.created_on)
@@ -101,7 +103,7 @@ class ActivitiesControllerTest < Redmine::ControllerTest
     get(
       :index,
       :params => {
-        :user_id => 299
+        :user_ids => [299]
       }
     )
     assert_response 404
@@ -113,7 +115,7 @@ class ActivitiesControllerTest < Redmine::ControllerTest
 
     @request.session[:user_id] = nil
     get :index, :params => {
-      :user_id => user.id
+      :user_ids => [user.id]
     }
 
     assert_response 404
@@ -199,12 +201,12 @@ class ActivitiesControllerTest < Redmine::ControllerTest
     get(
       :index,
       :params => {
-        :user_id => 2,
+        :user_ids => [2, 3],
         :format => 'atom'
       }
     )
     assert_response :success
-    assert_select 'title', :text => "Redmine: #{User.find(2).name}"
+    assert_select 'title', :text => "Redmine: #{User.find(2).name}, #{User.find(3).name}"
   end
 
   def test_index_atom_feed_with_subprojects